Low-carbon aluminum killed steel for high temperature applications



United States Patent 3,357,822 LOW-CAON ALUMINUM KILLED STEEL FOR HIGH TEMPERATURE APPLICATIONS Eiji Miyoshi and Takayasu Okada, Nishinomiya-shi,

Japan, assignors to Sumitomo Kinzoku Kogyo Kabushiki Kaisha, Osaka-shi, Japan No Drawing. Filed June 11, 1965, Ser. No. 463,367 Claims priority, application Japan, June 26, 1964, 39/ 36,570 8 Claims. (Cl. 75124) This invention relates to steel for high temperature applications which has good weldability and retains high strength at elevated temperatures with no loss of toughness.

Carbon steels for high-temperature uses are usually required to have high temperature strength, good weldability, and homogeneity. Especially in such applications as large pressure vessels, they are desired to have such other properties as fine grain size and sufliciently good toughness. At present the carbon steels available for high temperature applications include silicon-, silicon-aluminum, aluminum-killed steels and rimmed steels. Aluminum-killed steel has satisfactory weldability, uniformity in quality, fineness in grain size, and good toughness, but has such drawbacks as insufiicient high temperature strength and particularly poor creep characteristic due to the presence of a large amount of aluminum. Siliconaluminum-killed steel has adequate uniformity in quality, fine grain size, and high toughness, and yet lacks electric resistance seam weldability and has inferior high temperature characteristics because of the aluminum content. Silicon-killed steel in which the aluminum content is limited has such good high temperature strength and uniform quality that it has been put in practical applications as boiler tubes. Again, however, the presence of silicon deteriorate weldability of the steel, especially electric resistance seam weldability and grains bring poor toughness and is not adapted for special high-pressure Vessels for use at elevated temperature which call for good toughness. Furthermore, its high-temperature strength is lowered after stress relief annealing. In view of this, numerous studies have thus far been made for improvement of the weldability and workability of siliconkilled steel. Recently, methods have been proposed for the manufacture of low silicon and low carbon steels for high-temperature applications, whereby low carbon steels are subjected to vacuum treatment for the restriction of the silicon content. The low silicon, low carbon steels have satisfactory strength for use at high temperatures because the steels are subjected to vacuum treatment for killing so that the aluminum content is restricted. However, they have coarse grains and are not suitable for that reason for fabrication into high pressure vessels which call for good toughness. Moreover, they involve additional cost of vacuum treatment in the course of manufacture.

The present invention is intended to obviate these imperfections of the conventional carbon steel for high temperatures, and is concerned with steel having great high-temperature strength and adapted for mass production, which consists of carbon in the range of from 0.05 to 0.25%, silicon not more than 0.04%, manganese in the range of from 0.3 to 1.8%, aluminum from 0.005 to 0.040%, and nitrogen from 0.010 to 0.025%, a relation of (0.005 (N%)- /2(Al%)) being satisfied between nitrogen and aluminum, and the remainder sub- 3,357,822 Patented Dec. 12, 1967 stantially of iron. Since the steel according to the in vention is aluminum-killed steel, the ingot structure is homogeneous and the grain is thoroughly refined depending on the intended applications without any adverse effect upon the high temperature strength, Further, the weldability, in particular electric resistance weldability is improved, and thus it is made possible to manufacture seam welded pipes of Al-killed steel having good high temperature strength.

In general, aluminum is added to molten steel in order to obtain thorough deoxidation and fine grains. It is well known, however, that the addition of a large amount of aluminum tends to lower the high-temperature strength of the low carbon steel because the aluminum is combined with the nitrogen to form AlN, accompanied by a corresponding decrease in the amount of nitrogen in the form of solid solution, which has an important effect on the high-temperature strength of the product. After an intensive study on the subject, we found that, in adding nitrogen to aluminum-containing steel, the addition of nitrogen in excess to the amount precipitated as AlN in aluminum-containing low carbon steel will not necessarily increase the high-temperature strength of the product depending on the amount of silicon, if the melt is cooled slowly, but that the nitrogen gives a favorable effect upon the steel which is free from or contains only a negligible amount of silicon, exhibiting quite different behaviors with respect to the high-temperature strength attained in the presence of aluminum and nitrogen, depending on whether silicon is contained or not. These findings have further led to a conclusion, that, when the steel is deoxidized and refined by the addition of aluminum, steel sufficiently capable of use at elevated temperatures and which retains its high-temperature strength unaffected, can be manufactured, only if the steel is free from, or contains at most 0.04% of, silicon, and if an excess of nitrogen is added in such a manner that a relation (0.005 (N%)- /2(Al%)) holds between aluminum and nitrogen.

In comparison with conventional carbon steels for high temperature, the product according to the invention differs .from rimmed steels in the amounts of aluminum and nitrogen contained, and is also different from aluminumkilled steel in the amount of nitrogen. While the, steel manufactured under the invention must have its silicon content limited to 0.04% or less and must contain free nitrogen thereby to attain great high-temperature strength, it nevertheless dilfers from ordinary tough steels which contain high percentages of aluminum and nitrogen, in that the latter contains silicon and a large amount of nitrides such as AlN and takes advantage of the action of the precipitates in improving the toughness of the products.

The invention is illustrated by the following examples.

Test specimens of the steel manufactured in accordance with the invention (A1 through A6), Si-Al killed steels having excessive nitrogen contents (B, C,and D), Si-Al killed steels having excessive aluminum contents (E and F), and Si killed steels (G and H), all in the form of rods 18 mm. in diameter, were heated at 900 C. for one hour, and then air cooled, or cooled in the furnace at a rate of C./hr., or annealed at 700 C. for 50 hours following the furnace cooling. Thereafter, the specimens were subjected to a creep test at 450 C. under a load of 126 kg./crn. The test results and ferrite grain size are given in Table 1.

TABLE 1 Chemical analysis Creep test (450 (3., 12.6

kgjmmfi) Steel Ferrite spe:lmen Heat treatment Creeping rate grain,

501. Total Elonga Xlpercent hr. size 0 Si Mn P S Al N tion at 24/48 hr. 48/100 hr.

.41 0.13 0.03 0.00 0.015 0.021 0.010 0.0230 0 3: il7llolg %xo .1 4.; 0-7 .42 0.13 0.04 0.70 0.013 0.025 0.027 0.0232 3 11g. 100 c. 11 .2g 1%. 1g .3 "7 A3 0.13 0.00 0.40 0.011 0.003 0.017 0.0100 2 11. (100 6 711'): A4 0.13 0.04 0.50 0.008 0.010 0.007 0.0100 3 lk i'dbb filii'iI .44 15. .45 0.14 0.01 0.54 0.022 0.023 0.019 0.0182 3%: IgTdbEfif/HSI .2; 14.0 0.14 0.01 0.72 0.023 0.019 0.030 0.0285 900 c 1 F.'o.'('100'6. i 0.10 9.1 4.0

Chemical analysis Creep test (450 C., 12.6

kgJmmJ) Steel Ferrite specimen Heat treatment Creeping rate particle,

Sol. Total Eionga- X104 percent hr. size 0 Si M11 P S Al N tion at 24/43 hr. 48/100 hr.

0.13 0.13 0.45 0.012 0.019 0.017 0.0240 3%: (iogg ghg u 2.2g I 0.12 0.22 0.54 0.019 0.010 0.018 0.0170 900 0. 1 1 .0. (100 0.]11 5.23 229.1 0.12 0.12 0.40 0.010 0.012 0.012 0.0129 900 0. 1 F.C. 00 0.01 0.00 311.0 0.13 0.24 0.44 0.011 0.018 0.030 0.0140 900 0.x1 no. 100 0. 3.03 197.1 0.12 0.22 0.41 0.010 0.015 0.090 0.0202 900 0. 1 no. 00 0.01 7.00 304.0 0.15 0.10 0.39 0.010 0.020 0.002 0.0095 3 3: agap 0 g 0.15 0.14 0.50 0 012 0.020 0 003 0.0239 900 0. 1 F.C. (100 0. 0.095 900 0. 1 F.O. +700 c. 50 0.79 250.0

TABLE 2 Chemical composition (percent) Tensile properties Steel specimen Heat treatment Yield Tensile Elon- Contracpoint strength gation tion of C Si Mn Sol. Al N (kg/mm!) (kg/mm!) (percent) area (percent) 0.13 0.03 0.00 0. 010 0.0230 Ennacfi cooling (100 0.!11) 43.3 24.2 28.; 1 r coo ng 7. 0.13 0.04 0.76 0.027 0.0232 Furnace cooling (100 C./ 31.0 41.9 36.0 73.2 0.13. 0.03. 0.40 p 0. 017 0.0100 Furnace cooling (100 0.01). 25.0 30.5 38.0 72.0 0.12 0. 22 0. 54 0. 013 0. 0170 Furnace cooling (100 0.01.) 23. 4 40. 0 34. 8 09. 4 33. 0 45. 0 35. 3 73. 2 0.13 0.24 0. 40 0.002 21.1 38.2 37 00 25.3 43.2 28 70.3 o. 14 0. 08 0. 43 0. 00 15. 5 35.3 39 03 Air cooling.... 27. 4 41.1 41 72 From Table 1 it is manifest that the steel of the inven- Test specimens of the steel according to the invention tion has greater strength than ordinary Si-Al killed steels (Al through A6), Si-Al killed steel having an excessive withor without excessive nitrogen contents and that the nitrogen content (C), Si killed steel (I), and A1 killed presence or absence of a silicon has. an effect upon the steel (I) were. kept at 900 C. for one hour, and then high temperature, strength of the product. Furthermore, furnace cooled orair cooled. The tensile properties of it is clear that, whereas conventional Si killed steel and the specimens at room temperature tested were as shown Si-Al' killed steel lose some of their creep strength upon in Table 2.

hours of. annealing, the product. under the invention re- Test specimens of the steel according to the invention tains' great strength after slow cooling or annealing for (A), Si-Al killed steels (B and E), and Si killed steels alengthy period of time which. usually affects the creep (G and K) were kept at 900 C. for one hour and air propertyof steel to a, riiost serious extent, and. hence the cooled, followed or not followed by 10% cold working, steel manufactured according to the invention is stable or followed by the 10% cold working and annealing at witlia very low susceptibility to heat treatment. 250 C. for one hour and then with air cooling. The

transition temperatures (vTr 15) of the specimens (A), (B), (E), (G) and K) were as given in Table 3.

E? with respect to the maximum nitrogen content of 0.025% so that the above relation may be satisfied.

TABLE 3 Chem ca compos t on (percent) vTr 15 C-) Kept at 900 Steel specimen Kept at 900 C- Kept at 900 C for 1 hr, a r coo ed,

O S Mn 80 A N lor 1 hrand for 1 hr-, a r co ed, 0 d worked by an coo ed and c d worked annea ed at by 10% 250% 1'01 1 hr-, and an coo ed As will be seen from Table 3, the transition temperature of the steel according to the invention is similar to that of SiAl killed steel containing excess of nitrogen, but is lower than that of ordinary Si killed steel (G) by about 30 C.

The reason for limitation of the carbon content in the range from 0.05 to 0.25% in the composition of steel according to the invention is that the minimum of 0.05% is indispensable for maintaining the necessary tensile strength and a carbon content of over 0.25 will seriously affect the weldability and lower the toughness of the product and thus exert practically no favorable effect on the high-temperature strength of the resulting steel. In cases where particularly severe welding conditions must be satisfied, the maximum carbon content should be further confined to about 0.18% of the total amount. As above described, it is preferable that the silicon content should be kept as low as possible in order that the hightemperature strength of the steel where aluminum and nitrogen coexist be not affected thereby. In the present invention, a maximum of 0.04% is adopted as the amount which usually does not lower the high-temperature strength of materials to be heat treated. However, the upper limit must be lowered to 0.01% or less Where the materials are to be subjected to stress relief annealing. As an element for increasing high-temperature strengths and toughness of steel, manganese must be contained in an amount at least equivalent to 0.3% of the total amount. In an amount over 1.8%, it gives little effect on the hightemperature strength and adversely affects the weldability of steel. In cases where particularly rigorous welding conditions have to be met, the upper limit of the manganese content must be set at 1.4%. Nitrogen is an indispensable constitutent for giving sufficient high-temperature strength to the steel according to the invention, within the working temperature range thereof. Below 0.010%, it fails to bring such high-temperature strength. Generally the higher the nitrogen content the greater the high-temperature strength of the resulting steel. However, it is not advisable to use a nitrogen content exceeding 0.025% because it will cause liberation of gas upon solidification of the melt in the course of casting in the air, with consequent failure in killing.

Aluminum is added for the purpose of killing or grain refining. For the killing purpose, it needs about 0.005% of total amount of the melt, and for the refining purpose, about 0.01%. Below 0.005%, it will not bring a killed steel nor a homogeneous ingot. Above 0.010%, the grain size is refined and the toughness is effectively improved. In order to acquire sufiicient high-temperature strength in the case where nitrogen and aluminum coexist, it is essential that a relation 0.005 nitrogen /2 (aluminum should hold between the nitrogen content and the aluminum content. For this reason, an upper limit of 0.040% is herein chosen for the aluminum amount As described hereinbefore, the steel made in accordance with the invention is a killed steel which has good homogeneity, high-temperature strength, and good weldability, and is stable against hours of annealing such as stress relief annealing, and can be refined as according to the intended uses. With these advantages, it has very broad possibilities in the fields of seamless and seam welded steel pipes for boilers and large size high-pressure vessels.

What is claimed is:

1. A low carbon aluminum killed steel having great high-temperature strength which is adapted for use at elevated temperatures, consisting essentially of 0.05 to 0.25% of carbon, not more than 0.04% of silicon, 0.3 to 1.8% manganese, 0.005 to 0.040% of aluminum, 0.010 to 0.025 of nitrogen, said nitrogen and aluminum satisfying a relation 0.005 (nitrogen /2 (aluminum therebetween, and the remainder substantially of iron and incidental impurities.

2. A low carbon aluminum killed steel as defined in claim 1, wherein the silicon content is not more than 0.01%.

3. A low carbon aluminum killed steel having great high-temperature strength and good weldability which is adapted for use at elevated temperatures, consisting essentially of 0.05 to 0.25% of carbon, not more than 0.04% of silicon, 0.40 to 1.40% of manganese, 0.005 to 0.040% of aluminum, and 0.012 to 0.025% of nitrogen, said nitrogen and aluminum satisfying a relation 0.005 (nitrogen%) (aluminum therebetween, and the remainder substantially of iron and incidental impurities.

4. A low carbon aluminum killed steel as defined in claim 3, wherein the silicon content is not more than 0.01%.

5. A low carbon aluminum killed steel having great high-temperature strength, good weldability and toughness which is adapted for use at elevated temperatures, consisting essentially of 0.05 to 0.25% of carbon, not more than 0.04% of silicon, 0.40 to 1.40% of manganese, 0.01 to 0.04% of aluminum, 0.012 to 0.025% of nitrogen, said nitrogen and aluminum satisfying a relation 0.005 (nitrogen%) /2 (aluminum therebetween, and the remainder substantially of iron and incidental impurities.

6. A low carbon aluminum killed steel as defined in claim 5, wherein the silicon content is not more than 0.01%.

7. A low carbon aluminum killed steel as defined in claim 5', wherein the carbon content ranges from 0.05 to 0.18%.

8. A low carbon aluminum killed steel as defined in claim 6, wherein the carbon content ranges from 0.05 to 0.18%.

(References on following page) References Cited UNITED STATES PATENTS Murphy 75123 Rapatz 75123 Frazier 75124 Saunders 75123 X Naramura 75--124 8 Melloy 75123 Naramura 75124 Mueller 75124 X Nakamura 75124 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. A LOW CARBON ALUMINUM KILLED STEEL HAVING GREAT HIGH-TEMPERATURE STRENGTH WHICH IS ADAPTED FOR USE AT ELEVATED TEMPERATURES, CONSISTING ESSENTIALLY OF 0.05 TO 0.25% OF CARBON, NOT MORE THAN 0.04% OF SILICON, 0.3 TO 1.8% MAGANESE, 0.005 TO 0.040% OF ALUMINUM, 0.010 TO 0.025% OF NITROGEN, SAID NITROGEN AND ALUMINUM SATISFYING A RELATION 0.005<(NITROGEN%) -1/2 (ALUMINUM %) THEREBETWEEN, AND THE REMAINDER SUBSTANTIALLY OF IRON AND INCIDENTAL IMPURITIES. 